To solve for $x$ in the system of equations:

1. Substituting $y = -3$ into the equation $x + y = 9$:

$x + (-3) = 9$

2. Simplify:

$x - 3 = 9$

3. Add 3 to both sides:

$x = 12$

Solution:

$x = 12, \, y = -3$

Do you have any questions, or would you like further details?

Here are 5 related questions to explore:

1. How do you solve a system of linear equations with substitution?
2. What are the graphical interpretations of these solutions?
3. Can systems of equations have no solution or infinitely many solutions?
4. What happens if you substitute $y$ with another value?
5. How can we verify our solution?

Tip: Always check your solution by substituting the values back into the original equations.